An administration moderate was the likely source of a leak that outed a CIA spy and caused a national security scandal which threatened to bring down President George Bush's righthand man, it was claimed last night.

A book written by Newsweek journalists David Corn and Michael Isikoff claims the man who identified Valerie Plame as a spy in 2003 was former deputy secretary of state under Colin Powell, Richard Armitage, a man who enjoyed a gossip.

The leak sparked one of Washington's most intense political witchhunts. Karl Rove, the president's closest assistant, and Lewis "Scooter" Libby, vice-president Dick Cheney's former chief of staff, were both accused of identifying Valerie Plame in revenge for an article by her husband - former US ambassador Joseph Wilson - accusing Mr Bush of mishandling intelligence to justify the Iraq war.

"The initial leak, seized on by administration critics as evidence of how far the White House was willing to go to smear an opponent, came from a man who had no apparent intention of harming anyone," Isikoff writes in the magazine.

The revelation will shock observers in the capital, where charges of perjury and obstruction of justice are pending against Mr Libby, and Mr Rove's role in the affair is still under investigation.

Isikoff, the investigative correspondent of Newsweek, and Mr Corn, the magazine's Washington editor, say there were clues to Mr Armitage's identity in a second article by newspaper columnist Skip Novak months after the original October 2003 allegation.

Novak was the first to reveal Ms Plame as a CIA agent eight days after an article by her husband appeared in the New York Times accusing President Bush of misrepresenting information that Saddam Hussein had attempted to buy uranium from Niger for nuclear weapons. Mr Wilson was a diplomat in Africa for the Clinton administration.

Mr Novak said his source, who also spoke to Bob Woodward of the Washington Post, was "a senior administration official" who "was not a partisan gunslinger", but has consistently refused to reveal his identity.

An FBI investigation into the release of classified information followed, with agents heading for the White House in an effort to track down the mole. Mr Libby, also a national security adviser to the president, was later indicted by a grand jury.